Single point threading speed for aluminum

[o0norton0o]

I adapt a high tech motorcycle fork damper to fit my Norton Commando forks. I have to make a few different parts. I use the appropriate cutting speeds for those parts. The trickiest part I modify is the aluminum fork damper cap with it's integrated controls. The stock modern fork cap is wider than the Norton fork, but there's plenty of meat to turn it down and re-thread it to the Norton fork tube size threads (about 1-1/8" diameter x 20 tpi) I reduce the diameter of the fork cap at the proper calculated speed (about 1,700 rpm) but I slowed the rpm's down greatly to do the threading. The result was horrible and not a usable part at all. I can replace the fork cap nut pretty cheaply on ebay and give it another go, which is the plan, so,...

I have a question about threading aluminum and the choice of turning speed. As the speed of cutting moves toward the optimum speed for the best result, is there a mathematical progression in the increase of the quality of the cut? If I try, 450 rpm /900 rpm /1350rpm /1700rpm will I get a steadily increasing quality or is there a threshold based on the alloy or some other quality of aluminum that dictates the quality of threading. Is there something particularly about single point threading that requires consideration?

For example: I might be totally comfortable cutting down the diameter at 1,700 rpm's but not so comfortable threading at that speed. What are the solutions.

1) Suck it up and get comfortable at the 1,700 rpm speed cutting going away from the headstock with an upside down cutter where there's inches of safe space to stop the carriage.

2) Find a speed where things happen slow enough for you to feel comfortable and accept a slightly lower result

3) Something else?

*Feel free to mention anything that you feel I might be unaware of. Thank you in advance for any help

 

[stupoty]

I think the rpm for threading is dictated by your comfort level in not having a massive lathe crash.

Exceptions I can think of is for Hardinge lathes with the super clever high speed threading facility and CNC stuff.

Stu

 

[piper184]

I suspect that alloy content plays at least some part in the issue, as would heat treatment?
Did you try really light cuts when using the speeds you are comfortable with?
Threading is usually a much faster feed rate than general cutting so even at a given RPM the tool would experience different forces than regular turning cuts.
At least now you have some "scrap" to experiment with.

 

[pdentrem]

What lube were you using? Tool freshly sharpened?

 

[Eddyde]

If you are using carbide, try HSS instead. Check your center height and use lube, WD40 works well.

 

[OTmachine]

Unless you are doing CNC, What is your hurry? Even then, 450 rpm threading would be hard on the machine, IMHO.

 

[o0norton0o]

I suspect that alloy content plays at least some part in the issue, as would heat treatment?
Did you try really light cuts when using the speeds you are comfortable with?
Threading is usually a much faster feed rate than general cutting so even at a given RPM the tool would experience different forces than regular turning cuts.
At least now you have some "scrap" to experiment with.

ha ha, yeah I have another scrap piece to experiment with. that will keep me working towards a solution without ruining another piece

 

[Just for fun]

If you're going to cut thread at 1,700 RPM, I would thread away from the chuck. If it was me and I was threading toward the chuck I would take shallow cuts and go at it at about 75 RPM.

 

[higgite]

I’ve cut perfectly good threads in 6061-T6 at 100 rpm using an HSS tool and some lube. It would help us to know your equipment and procedure. Lathe size? Aluminum alloy being threaded? RPM? Threading tool HSS or carbide? Approach angle of tool to work piece? DOC for each pass? Pics of mucked up threads?

Tom

 

[o0norton0o]

What lube were you using? Tool freshly sharpened?

I was using 20wt non detergent oil and I tried the carbide first then switched to HSS

If you are using carbide, try HSS instead. Check your center height and use lube, WD40 works well.

I did switch once I started not liking the result

Unless you are doing CNC, What is your hurry? Even then, 450 rpm threading would be hard on the machine, IMHO.

Well, it was much worse than that. I was turning at 55 rpm. I had forgotten about factoring in cutting speed and even though I had made these once before, I had forgotten what speed I had previously used, I realized too late for this part that turning speed was my issue, and the 20 tpi was so fine that I kept making passes waiting for the threads to begin emerging, but they just didn't.

Eventually, I realized that I had made the draw bar mechanism to lock my chuck in place for reverse chuck operation and had gone through a whole series of experiments to rig up a reverse direction switch, but didn't have the confidence to thread at higher speed going in reverse with an upside down cutter, going away from the headstock......